Calcium-dependent changes in postsynaptic densities (PSDs) may be involved in activity-dependent modification of synaptic function. The most abundant protein in PSDs is similar or identical to the alpha~subunit of calcium/calmodulin-dependent protein kinase II. We studied the autophosphorylation characteristics of the associated calcium /calmodulin-dependent protein kinase (CaM kinase) in isolated PSDs. When endogenous phosphatase is inhibited, the PSD-associated kinase, like its cytosolic counterpart, autophosphorylates in the presence of calcium, or in the absence of calcium, following brief incubation with the ion. Sequencing of phosphopeptides indicates that a major site of phosphorylation under both conditions is a threonine residue corresponding to T-253 of the alpha-CaM kinase II sequence. This mode of regulation is unique to the PSD-associated enzyme, since the soluble CaM kinase is described to be autophosphorylated at this site only at low concentrations of ATP and only in the presence of calcium. Differences in phosphopeptide patterns generated from the kinase autophosphorylated in the presence and absence of calcium indicate the presence of additional sites autophosphorylated under one condition only. Thus, PSD- associated CaM kinase, through its intricate, multisite autophosphorylation properties, may be capable of recording fluctuations of calcium. Studies on structural changes in PSDs mediated by another calcium-dependent enzyme, calpain, were continued. Electron microscopy of single and serial sections from freeze-substituted samples indicate that, following limited proteolysis that caused extensive degradation of spectrin but left most other major components intact, the central lamina of the PSD looked less dense and widened, as if it had unraveled. It is likely that these changes are due to breakdown of spectrin-mediated cross-bridges. Such a mechanism might serve to expose occluded sites in PSDs.